Technical Field
The present disclosure relates to a high-dynamic-range pixel, that is, a pixel capable of providing a signal representative of the light for a wide range of light intensity levels.
Discussion of the Related Art
FIG. 1 is a copy of FIG. 1 of U.S. Pat. No. 8,513,761 (B9164) of the applicant and schematically illustrates an example of an image sensor pixel circuit.
The pixel comprises a photodiode D having a first pole, here the anode, connected to ground GND, and having a second pole, here the cathode, coupled to a sense node S by a transfer N-channel MOS transistor TR. Sense node S is coupled to a high reference potential, for example, power supply potential Vdd, by a precharge MOS transistor RST. Pixel 1 is associated with a read circuit of node S comprising a MOS transistor SF, assembled as a source follower, and a selection MOS transistor RD. The gate of transistor SF is connected to node S, the drain of transistor SF is connected to power supply potential Vdd, and the source of transistor SF is connected to the drain of transistor RD, the source of transistor RD being connected by a terminal P of a processing circuit (not shown). It should be noted that a plurality of pixels, currently four pixels, may be associated with a same read circuit. Generally, the gate control signals of transistors RD, RST, and TR are provided by control circuits, not shown in FIG. 1.
In operation, during an illumination phase, or integration phase, transfer transistor TR is set to the non-conductive state. The light received by the pixel causes the generation of electron-hole pairs in photodiode D, the electrons being stored in the photodiode. During a read phase, precharge transistor RST, initially in the on state, is set to the non-conductive state. The potential of node S is then substantially equal to Vdd. Transistor TR is then set to the on state and then to the off state, which causes the transfer of the photogenerated electrons stored in photodiode D to sense node S. The potential variation of sense node S resulting from the electron transfer is read by the associated read circuit and the quantity of light received by this pixel during the integration phase is deduced therefrom.
The dynamic range of the pixel of FIG. 1 corresponds to the maximum quantity of light that the pixel can detect. Such a maximum quantity of light depends on the maximum quantity of electrons that photodiode D can store, and thus, in particular, on the dimensions of photodiode D. Thus, when pixels of relatively smaller dimensions are desired to be formed, the dimensions of photodiode D are decreased, which results in a decrease in the pixel dynamic range.
It would thus be desirable to have a pixel enabling to store more photogenerated charges than a pixel of the type of that in FIG. 1. It would also be desirable for this larger storage capacity not to require increasing the pixel dimensions.